To me, the meaning here is that the speaker is mourning the death of Pres. Kennedy. He is asking God, presumably, why this was allowed to happen.
The middle two stanzas, I think, are saying that JFK's legacy will depend on what people do -- if it inspires people to do great things, then it was a meaningful death. If not, maybe it wasn't.
I suppose that is why joy and praise are involved here -- because we celebrate his life and his legacy.
For me, this does not really make me think much because JFK was before my time and I don't think he was all that special as a president.
So all it really does for me is to show me how much some people in the '60s thought of JFK.
First, start with the title. The poem is an elegy, and elegies are written in response to the death of a person or being. So the poem is striving to be for the remembrance of JFK, the 35th president of the United States. One does not have to be a fan or like JFK to appreciate this poem.
In the first stanza, the speaker of the poem is asking several questions, questions that are often asked in reference to anyone who has died. Mourners look to the heavens for answers to these questions and, as the speaker writes, "The heavens are silent" (3).
In stanzas two and three the speaker grapples with how to remember a person who has died. Is is often the case that we say that a person and a person's memory live on within the living, and that is what the speaker is asking us to do, as "How we choose to live/ Will decide its meaning" (8-9).
The last stanza has the speaker coming to terms with the gamut of human emotions surrounding death: the dichotomies, the speaker argues, are one in the same.
I think this poem has a certain universality to it. It can and should be appreciated by all, even those who may not know JFK. The poem is about remembrance and lament over a person's death.